andrews 24/39, Installation Manual, Operation And Service Manual

Page: 80 / 96

APPENDICES
80
°C 0 1 2 3 4 5 6 7 8 9
4.5 26109 25400 24712 24045 23399 22771 22163 21573 21000 20445
10 19906 19383 18876 18383 17905 17440 16990 16553 16128 15715
15.5 15314 14925 14548 14180 13823 13477 13140 12812 12494 12185
21.1 11884 11592 11308 11032 10763 10502 10248 1000 9760 9526
26.7 9299 9078 8862 8653 8449 8250 8057 7869 7685 7507
32 7333 7165 7000 6839 6683 6531 6383 6238 6098 5961
37 5827 5697 5570 5446 5326 5208 5094 4982 4873 4767
44 4663 4562 4464 4368 4274 4183 4094 4006 3922 3839
49 3758 3679 3602 3527 3453 3382 3312 3244 3177 3112
55 3048 2986 2925 2866 2808 2752 2697 2643 2590 2538
60 2488 2439 2391 2344 2298 2253 2209 2166 2124 2083
65.5 2043 2004 1966 1928 1891 1856 1820 1786 1753 1720
71 1688 1656 1625 1595 1566 1537 1509 1481 1454 1427
76 1402 1376 1351 1327 1303 1280 1257 1235 1213 1191
82 1170 1150 1129 1110 1090 1071 1053 1035 1017 999
88 9 82 965 949 933 917 901 886 871 857 842
93 828 814 801 788 775 762 749 737 725 713
In Degrees C
Sensor Resistance at Various Temperatures
Example: If water temperature is 26.7
°
C, then the resistance through the sensor would be 8449 (see shaded area).
NOTE: Sensor resistance increases as the temperature falls.
Page 26
SERVICE PROCEDURE D24-I
Thermostat Circuit Testing
Upper thermister location
(applicable models)
Lower thermister
access located inside
control box.
Sensor Resistance Testing
Upper Sensor
1. Determine resistance value of upper sensor. Test across
grey wires.
2. Draw quart of water off T&P valv e. Using a thermometer,
determine water temperature.
3. Use table below to verify correct resistance per water
temperature measured.
Lower Sensor:
1. Determine resistance value of lower sensor. Test across
center wire (common) to each outside wire. Resistance of both
thermisters in the lower sensor should be close to each other.
If the resistance values for both thermisters are not close to
each other, replace the lower sensor. The dual thermisters are
used to provide high limit protection in case the thermostat
circuit fails to shut off the water heater.
2. Draw quart of water off
Drain Valve.
Using a thermometer, determine water temperature.
3. Use table below to verify correct resistance per water
temperature measured.
Are readings
correct?
N
Replace
thermister
Y
Check harness
continuity.
Continuity okay?
Y
Replace
control board
Replace
harness
N
CAUTION
Be Carefu l When Making Resi st ance
Measurem ents Not to Damage or
Deform Conne ct or s o r Co nne ct or Pi ns .
Conditions: Upper or Lower Sensor Reading
Faulty, High Water Temperature, or suspect
thermostat is not accurate.
Checking resistance of
upper sensor
Checking resistance of
lower sensor (center and
outside pins)
10
11